Methods and apparatus for removing residual monomers

ABSTRACT

An apparatus and a method for removing residual monomers from a slurry containing a polyvinyl chloride. The apparatus has a cylindrical tower with plural perforated plates provided in a vertical direction in the tower, plural chambers formed on the perforated plates serving as their bottom surface, slurry introducing portions provided at two or more of the chambers, flow-down sections provided between the perforated plates to allow the slurry to sequentially flow down from the perforated plate of an upper chamber to the perforated plate of a lower chamber, a steam introducing port at a bottom portion of the tower, a deaerating port at a top portion of the tower, a slurry discharging port at a chamber below a chamber having the slurry introducing port, and hot water ejectors directly under the perforated plates directed at least toward the lower surface of the perforated plates. The apparatus is constructed so that the slurry introducing pipe is connected to a slurry introducing port through an on-off valve and the inside diameter of the slurry introducing pipe is progressively enlarged to 1.2 times or more toward the slurry introducing port.

The present invention relates to a method for removing residual monomersand an apparatus for removing residual monomers using the method. Morespecifically, the invention relates to a method and an apparatus forremoving residual monomers by which unreacted residual monomers mainlycomprising vinyl chloride monomers (hereinafter abbreviated to VCM)which remain in vinyl chloride resin (hereinafter abbreviated to PVC)particles and an aqueous medium when particularly a PVC is produced areremoved.

BACKGROUND ART

While PVC is generally produced by suspension polymerization method,emulsion polymerization method, or bulk polymerization method,particularly a suspension polymerization method and emulsionpolymerization method have widely been used from the advantages that thereaction heat can readily be removed and products in which the amount ofimpurities is small can be obtained.

Suspension polymerization method and emulsion polymerization method areusually conducted by charging VCM together with an aqueous medium,dispersing agent or emulsifier, and polymerization initiator into apolymerization vessel provided with a stirrer, and polymerizing the VCMwhile stirring and maintaining it at a predetermined temperature.

Usually, polymerization reaction is not continued to the point where VCMis converted by 100% to PVC, and it is terminated at a stage whereproduction efficiency is high, that is, terminated at a polymerizationconversion of 80 to 95%. After termination of the polymerizationreaction, residual monomers in a polymerization vessel are separatedfrom a PVC slurry (mixed dispersion comprising mainly PVC particles andan aqueous medium) and then recovered. However, the PVC slurry generallycontains a few % of unreacted residual monomers.

Subsequently, the aqueous medium in the PVC slurry is mechanicallyseparated, and the residue is dried by a hot-air drying or one ofvarious other methods to form PVC powders. On this occasion, VCM iscontained in the separated aqueous medium described above, exhaust airin a hot-air drying, and the PVC powders each in an extent which is madeinto a matter of concern on the grounds of environmental sanitation, orclearly considered to be harmful.

Various methods have been proposed to completely remove the dischargesformed in such production and VCM in the PVC powders, or to decrease thecontent of VCM down to the extent at which it is harmless toenvironmental sanitation.

As the method for removing and recovering unreacted residual monomersmore efficiently, methods for removing and recovering residual monomersfrom a PVC slurry by using a treating tower which has plural trays madeof perforated plates therein and has a steam ejecting port at its bottomportion were proposed (Laid-open Japanese Patent Publication No. Sho54-8693 and Laid-open Japanese Patent Publication No. Sho 56-22305).

Characteristics of these methods are trays made of perforated plates inwhich the base are constructed by perforated plates and partition wallsare installed on the perforated plates so that treating passages areformed in a zigzag; a PVC slurry is exposed to steam ejected from alower portion through perforations of the perforated plates, during thetime when the PVC slurry flows along the treatment passages on the traysmade of perforated plates, and residual monomers contained in the PVCslurry are evaporated and separated.

Further, a method and an apparatus for removing residual monomers wereproposed in which bubbling caused in the upper section of a tower issuppressed by making its diameter larger than that of the lower sectionof the tower, thereby stabilized operation and prevention ofincorporation of degradated particles formed by the bubbling becomepossible (Laid-open Japanese Patent Publication No. Hei 07-224109).

Contact time of the slurry with steam necessary for removing VCM in aPVC slurry differs according to the grade of PVC. Generally, a slurry ofa PVC having a low polymerization degree is hardly demonomerized and aslurry of a PVC having a high polymerization degree is readilydemonomerized.

However, in the methods described above, it is impossible to efficientlytreat different grade of PVC slurries having different contact timeswith steam necessary for removing residual monomers in the sameapparatus because residence time of PVC slurries is maintained constantby the treating passages and partition walls on the trays made ofperforated plates.

That is, when a PVC slurry from which residual monomers are difficultlyremoved is treated in an apparatus which is designed for treating a PVCslurry from which residual monomers can readily be removed, contact timewith steam is insufficient, and unreacted residual monomers can notsufficiently be removed from the PVC slurry. Conversely, when a PVCslurry from which residual monomers can readily be removed is treated inan apparatus which is designed for treating a PVC slurry from whichresidual monomers are difficultly removed, PVC particles contact withsteam for a time longer than necessary after the residual monomers wereremoved, and thermal degradation of the PVC is caused, and quality ofPVC products deteriorates.

In the plants for manufacturing PVC, usually many times a plural gradesare produced by the same facility. Accordingly, if PVC powders remain inthe equipment when a product grade is changed to a different one, PVCparticles of different grades are incorporated to cause an inconveniencesuch as fish eyes, thereby to depreciate the value of products.

Even in the case where the same grade is treated, if PVC particlesadhere within a tower for removing residual monomers and remain for longperiod of time, the particles cause thermal degradation by steam, andthe particles unwillingly discolor brown (hereinafter, discoloration ofparticles by thermal degradation is referred to as coloration). Ifcolored PVC particles adhered within the tower are fell off the wallsurface of the tower and mixed in a PVC slurry, the value of products isreduced at the time of their processing.

In addition, in the method of demonomerization described above in whichbubbling is suppressed, a PVC slurry disperses when it is introduced ina tower from a PVC slurry introducing portion, adheres on the internalwall of the tower, and causes such a problem of colored PVC particles asdescribed above, whereas the residence degradation of a part of PVC canbe prevented.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an apparatus and amethod for removing residual monomers from a slurry containing apolyvinyl chloride and residual monomers after polymerization by whichapparatus and method adhesion of PVC particles on the internal wall ofthe tower can be averted by preventing the dispersion of slurry in atower for removing residual monomers, formation of a PVC containing ahigh concentration of residual monomers due to insufficient treatmentcan be precluded by adjusting the treating time to a suitable one foreach PVC slurry to be treated, degradation of the PVC by an excessivetreatment with steam can be avoided, and quality deterioration due tomixing of different grade of slurries can be prevented, when residualmonomers contained in the slurry are removed by a treatment with steamin the case where plural products having different properties areproduced in the same equipment in turn.

As a result of diligent investigations by the present inventors, amethod and an apparatus for removing residual monomers have been foundby which residence time of a PVC slurry in an apparatus for removingresidual monomers can be adjusted so as to be suitable for a pluralgrade of PVC slurries, dispersion of a PVC slurry at a slurryintroducing portion can be prevented, PVC powders do not remain in atreating apparatus when a grade of slurry is changed to another grade,and PVC particles do not adhere on the internal wall of an apparatus, inan introducing pipe particularly at from a slurry introducing port to anon-off valve which tends to become a dead angle, and further in theon-off valve and a slurry introducing port.

Accordingly, the present invention is summarized as follows:

(1) A method for removing residual monomers from a slurry containing apolyvinyl chloride and residual monomers after polymerization by usingan apparatus comprising a cylindrical tower which has plural perforatedplates provided in a vertical direction in the tower, plural chambersformed respectively on the perforated plates serving as the bottomsurface of the chambers, slurry introducing portions providedrespectively at two or more of the chambers described above, flow-downsections such as downcomers provided between the perforated plates so asto allow the slurry to sequentially flow down from the perforated plateof an upper chamber to the perforated plate of a lower chamber, a steamintroducing port provided at a bottom portion of the tower, a deaeratingport provided at a top portion of the tower, a slurry discharging portprovided at a chamber positioned lower than the chambers having theslurry introducing portion described above, and a hot water ejectingmeans provided directly under the perforated plates described above anddirected toward at least the lower surface of the perforated platesdescribed above characterized in that the amount of the slurryintroduced is 0.1 to 300 m³/h per 1 m² of the area of the perforatedplate serving as the bottom surface of the chamber described above andthe linear velocity of the slurry at the slurry introducing port of thechamber described above is 6 m/sec or lower when the slurry isintroduced from the slurry introducing portion of the chamber describedabove.

(2) An apparatus for removing residual monomers from a slurry containinga polyvinyl chloride and residual monomers after polymerizationcharacterized in that the apparatus comprises a cylindrical tower whichhas plural perforated plates provided in a vertical direction in thetower, plural chambers formed respectively on the perforated platesserving as the bottom surface of the chambers, slurry introducingportions provided at two or more of the chambers described above,flow-down sections such as downcomers provided between the perforatedplates so as to allow the slurry to sequentially flow down from theperforated plate of an upper chamber to the perforated plate of a lowerchamber, a steam introducing port provided at a bottom portion of thetower, a deaerating port provided at a top portion of the tower, aslurry discharging port provided at a chamber positioned lower than thechambers having the slurry introducing port described above, and a hotwater ejecting means provided directly under the perforated platesdescribed above and directed toward at least the lower surface of theperforated plates described above, the slurry introducing portionsdescribed above comprising slurry introducing ports provided at thetower described above and slurry introducing pipes connected to theslurry introducing ports, and the inside diameter of the slurryintroducing pipes being enlarged to 1.2 times or more toward the slurryintroducing ports.

(3) The apparatus for removing residual monomers recited in paragraph(2) above wherein an on-off valve is provided in the slurry introducingportion provided in a chamber positioned below the uppermost chamber.

(4) The apparatus for removing residual monomers recited in paragraph(3) above wherein the on-off valve in the slurry introducing portion isa ball valve.

(5) The apparatus for removing residual monomers recited in paragraph(3) above wherein a hot water ejecting means is provided between theslurry introducing port and the on-off valve in the slurry introducingportion with the hot water ejecting means being directed toward theon-off valve.

In the case where a PVC slurry which requires a long time forevaporating and separating residual monomers is treated according to thepresent invention, it is possible to secure a residence time andsufficiently remove residual monomers by introducing a PVC slurry from aslurry introducing portion at an upper chamber, since a PVC slurryintroducing portion is provided at optional two or more chambers in anapparatus for removing residual monomers provided with trays made ofperforated plates. On the other hand, when a PVC slurry from whichresidual monomers can be evaporated off in a short period of time istreated, heat degradation of PVC resin can be prevented and the monomersare satisfactorily removed by introducing a PVC slurry from a slurryintroducing portion at a lower chamber since VCM can sufficiently beevaporated and separated in a short residence time.

In the present invention, a slurry is introduced so that its linearspeed at a slurry introducing port is 6 m/sec or lower, and removal ofresidual monomers is carried out.

Therefore, the apparatus of the present invention preferably has suchmechanisms as follows:

a) a slurry introducing portion is provided between a slurry introducingpipe and a slurry introducing port,

b) the inside diameter of a slurry introducing portion at the side ofthe slurry introducing port is larger than that of the slurryintroducing portion at the side of the slurry introducing pipe,

c) the inside diameter of the slurry introducing portion is increased asit gets closer to the slurry introducing port, and

d) the increase of the inside diameter is decided so that the linearspeed of a slurry is maintained at 6 m/sec or lower (preferably 5 m/secor lower) even if the volume of the slurry was increased by thedifference in temperature and pressure at between the slurry introducingportion and slurry introducing port. Such ratio of pipe diameter is 1.2or more, preferably 1.5 to 3.5, and more desirably 1.5 to 2.0.

Whereas the slurry (at a temperature of 50 to 150° C.) introduced from aslurry introducing portion is in a pressurized condition (0.5 to 10kg/cm²), the VCM contained in the slurry is vaporized and its volume issuddenly expanded at the time of slurry introduction since thetemperature inside the tower is higher than that of the slurry in theslurry introducing pipe (usual temperature difference is 5 to 50° C.).At this time, if the diameters of the slurry introducing pipe and slurryintroducing port of the tower were the same, the slurry comes todisperse on the internal wall of the tower by the expansion of volumeand flow velocity by transportation with a pump. In order to avoid this,the volume corresponding to the expansion of slurry volume is secured bythe space formed by the increase in the diameter of the pipe coming tothe slurry introducing port, and the linear speed of slurry at theslurry introducing port is adjusted to 6 m/sec or lower and preferably 5to 1 m/sec. By this, the dispersion of a slurry specifically due to theinstantaneous volume expansion of a dissolved gas can be avoided, andthe coloration of PVC particles adhered on the internal wall of a towerdue to their exposure to steam for long time can be prevented. Slurryintroducing port to be provided at a tower is desirably located at aposition higher than the slurry surface in the chamber at which theslurry introducing port is provided. It is desirable that the diameterof a slurry introducing pipe is gradually increased so that flow of aslurry is not disturbed. Forward portion of a slurry introducing pipethan the portion where its diameter is increased is required to secure aslurry flow speed sufficient to flow the slurry by making its diametersmaller than that of a slurry introducing port so that PVC particles ina slurry do not deposit at a horizontal portion of the pipe and theblockage of the pipe does not occur.

On-off valve is desirably provided in a slurry introducing portion whichis provided at a chamber located at a lower position than the uppermostchamber. The on-off valve provided in the slurry introducing pipe isused when one of a plurality of slurry introducing portions in ademonomerizing tower is selected and a slurry is introducedtherethrough, in other words, when a slurry introducing portion to beused is selected depending on the demonomerization characteristic of aslurry. That is, a slurry can be supplied from a selected slurryintroducing portion to a demonomerizing tower by opening the on-offvalve in the selected slurry introducing portion and closing the on-offvalve in other slurry introducing portions. As such on-off valve, a ballvalve is preferable. It can be avoided by providing an on-off valve ineach of a plurality of slurry introducing portions as described abovethat a residual resin contained in a slurry previously treated mixesfrom an introducing pipe which is not currently being used and that aslurry which is presently being treated incorporates in an introducingpipe which is not presently being used and becomes contamination at thetime when this pipe is used next time.

It is preferable to provide a hot water ejecting apparatus between aslurry introducing port and an on-off valve in a slurry introducingportion with the apparatus being directed toward at least the on-offvalve, and to wash the on-off valve, and the pipe portion between theon-off valve and slurry introducing port, when necessary. The slurryintroducing port is preferably positioned as close as possible to theon-off valve so that the portion between them is easy to wash. Thiswashing apparatus employing hot water is used, for example, during thetime when a slurry is being introduced from a PVC slurry introducingportion and a demonomerizing treatment is being conducted, to wash otherPVC slurry introducing portions which are not being used for introducinga PVC slurry. By such method, occurrence of colored particles due to along stay of PVC particles at a PVC slurry introducing portion canefficiently avoided, and staying of PVC particles in the PVC slurryintroducing portion at the time of changing grade can be prevented toavoid mixing of a different grade.

In the present invention, PVC means homopolymers of VCM, copolymers ofVCM with a polymerizable monomer which can polymerize with VCM, polymersprepared by grafting VCM to olefin polymers or the like, and polymercompositions comprising two or more those polymers. In order toefficiently remove residual monomers according to the present invention,polymers containing 50% by weight or more of VCM as polymer constitutingunit are preferable.

Method for obtaining the polymers may be suspension polymerization oremulsion polymerization.

As polymerizable monomer which can polymerize with VCM, carboxylic acidesters of vinyl alcohol such as vinyl acetate, vinyl ethers such as analkylvinyl ether, unsaturated carboxylic acid esters such as acrylatesand methacrylates, vinylidene halides such as vinylidene chloride andvinylidene fluoride, unsaturated nitriles such as acrylonitrile, andolefins such as ethylene and propylene can specifically be mentioned.

Since, in polymerization reactions, a dispersing agent such as polyvinylalcohol and hydroxypropylmethyl cellulose and an emulsifier such assodium alkylsulfate and sodium alkylsulfonate are used; and a buffer,particle diameter regulating agent, depressor for scale build up,defoaming agent, and the like are further used when necessary, a minoramount of these additives are sometimes mixed in a PVC slurry.

PVC slurries treated according to the present invention preferably havea concentration of PVC dispersed in the PVC slurries, that is, slurryconcentration of 5 to 45% by weight, and the concentration is moredesirably 10 to 40% by weight. When the slurry concentration is toohigh, flowablity of the PVC slurry in the tower becomes worse. On theother hand, when it is too low, efficiency of removing treatment lowers.

Whereas a PVC slurry to be treated according to the present invention istransferred into a PVC slurry tank after polymerization reaction wasterminated, unreacted VCM was released by the internal pressure of thepolymerization vessel and recovered, and then the internal pressure wasreduced down to normal atmospheric pressure, the PVC slurry may betransferred into a PVC slurry tank before the pressure in thepolymerization vessel is reduced to a normal atmospheric pressure, or aPVC slurry in the middle of polymerization may be transferred into thetank at the time when the polymerization was terminated at an optionalpolymerization conversion.

The PVC slurry transferred into a PVC slurry tank is introduced into anapparatus of the present invention for removing residual monomers at apredetermined flow rate by using a pump.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing an apparatus of the presentinvention for removing residual monomers.

FIG. 2 is a schematic plan view of a perforated plate.

FIG. 3 is a schematic vertical cross-sectional view of a portion forintroducing a PVC slurry onto a perforated plate.

FIG. 4 is a schematic vertical cross-sectional view of a PVC slurryintroducing portion.

BEST MODE FOR CARRYING OUT THE INVENTION

An apparatus which can preferably be used in the present invention forremoving residual monomers and a process for removing residual monomersfrom a PVC slurry by employing the apparatus are specifically describedwith reference to FIGS. 1 to 4. However, the present invention is by nomeans restricted to such specific description.

FIG. 1 is a schematic drawing showing an apparatus for removing residualmonomers, FIG. 2 is a schematic plan view of a plate, and FIGS. 3 and 4are schematic vertical cross-sectional views of a PVC slurry introducingportion for introducing the slurry onto a perforated plate.

In FIG. 1, an apparatus 4 for removing residual monomers is constructedso that it comprises a cylindrical tower 46, plural perforated plates 32to 38 provided in a vertical direction in the tower, a plurality ofchambers formed respectively on the perforated plates serving as bottomsurface of the chambers, slurry introducing devices 19 to 24 provided attwo or more of the chambers described above, flow-down sections such asdowncomers 13 to 18 provided between the perforated plates so as toallow the slurry to sequentially flow down from the perforated plate ofan upper chamber to the perforated plate of a lower chamber, a steamintroducing port 10 provided at a bottom portion of the tower, adeaerating port 11 provided at a top portion of the tower, a slurrydischarging port 12 provided at a chamber positioned lower than thechambers having the slurry introducing devices described above, a hotwater ejecting means 25 provided at the ceiling portion 8 in the towerand directed to the ceiling of the tower, and hot water ejecting means26 to 31 provided directly under the perforated plates described aboveand directed toward at least the lower surface of the perforated platesdescribed above. In the apparatus, a slurry introducing pipe 48 isconnected to a slurry introducing port 50 through an on-off valve 49 asshown in FIGS. 3 and 4. And the inside diameter of the slurryintroducing pipe 48 is enlarged so that it becomes 1.2 times or more (inthe Figure, D/d≧1.2) in the direction towards the slurry introducingport 50.

PVC slurry obtained by a suspension polymerization or emulsionpolymerization and temporarily stored in a PVC slurry tank 1 is led to aheat exchanger 3 with a pump 2, heated to a predetermined temperature inthe heat exchanger 3, and then introduced into a tower from optional PVCslurry introducing portions 19 to 24 of an apparatus 4 for removingresidual monomers. Whereas an on-off valve is not provided in slurryintroducing portion 19 at the uppermost chamber, it may be provided whennecessary.

Transport amount of a pump is preferably adjusted so that the flow rateof a PVC slurry introduced in a tower is 0.1 to 300 m³/h (morepreferably 1 to 100 m³/h) per 1 m² of the area of perforated plate 47shown in FIG. 2.

PVC slurry introduced in a tower is desirably in a condition preheatedat 50 to 100° C. with heat exchanger 3. Efficiency of removing residualmonomers is improved by this preheating.

Inside diameter of tower 46 is 200 to 10,000 mm, and the height of thetower is 2 to 20 times, preferably 5 to 15 times relative to the insidediameter. Also, inside diameter of each chamber in the tower may bedifferent when necessary.

The space delimited with the bottom of a tower and a perforated plate,with a perforated plate and another perforated plate located atimmediately upper position, or with a perforated plate and the top of atower in apparatus 4 for removing residual monomers is referred to aschamber. Number of chambers necessary for treating residual monomers isdecided by residence time required when residual monomers are removedfrom a PVC slurry.

Difficulty of removing monomers from a PVC slurry is caused by thestructure of PVC particles in a PVC slurry. When the ratio of the volumeof micropores in PVC particles is large, contact of PVC particles withsteam is favorable and the particles are readily demonomerized, but whenthe ratio of the volume of micropores is small, demonomerization becomesdifficult.

Residence time of a PVC slurry in a tower is decided by such difficultyof demonomerization from a PVC slurry as described above, concentrationof residual monomers contained in a PVC slurry introduced into a tower,and a settings value of the concentration of residual monomers after thetreatment at PVC slurry discharging port 12.

When the residence time of a PVC slurry in a tower is long, residualmonomers can be removed at a high level from PVC particles existing in aPVC slurry. However, if the residence time is too long, coloring of PVCparticles due to thermal degradation is unwillingly caused. Accordingly,contact of a PVC slurry with steam for a longer time than necessary isnot preferable. Thus, the residence time is necessary to be adjustedwith reference to the difficulty of demonomerization from a PVC slurry.

It is desirable to construct an apparatus used in the present inventionso that a slurry introducing portion is provided at plural chambersdisposed in a vertical direction in a tower, and for instance, a PVCslurry from which residual monomers are very difficultly removed isintroduced from a PVC slurry introducing portion at an upper portion ofa tower and a PVC slurry from which residual monomers are readilyremoved is introduced from a slurry introducing portion at a lowerportion of a tower. Taking a PVC slurry from which residual monomers arevery difficultly removed, as an example, the slurry is introduced into atower from slurry introducing portion 19 at an upper portion of thetower. The slurry introduced is passes through treating passages formedby partition walls 39 to 44 on perforated plate 32 and tower 46,overflows partition board 45 (FIG. 3), and flows onto perforated plate33 through downcomer 13. Subsequently, the slurry introduced onperforated plate 33 passes through treating passages on perforated plate33 and further flows onto perforated plate 34 located under perforatedplate 33 through downcomer 14. After passing through treating passageson perforated plates 32 to 38 in such a way as described above, theslurry is discharged outside a tower from PVC slurry discharging port 12provided at the bottom chamber 52 of the tower. Also, in the case of aPVC slurry from which residual monomers are a little difficultlyremoved, for instance, the slurry is introduced from PVC slurryintroducing portion 21, and passed through treating passages onperforated plates 34 to 38 to shorten residence time. In a like manner,in the case of a PVC slurry of a good demonomerization ability, forinstance, a slurry is introduced from slurry introducing portion 23 tofurther shorten the residence time, and thus thermal degradation due tocontact with steam more than necessary can be avoided.

Slurry introducing portion from which a PVC slurry is introducedcomprises PVC slurry introducing pipe 48, on-off valve 49, and PVCslurry introducing port 50 as shown in FIG. 3, and the inside diameterof PVC slurry introducing pipe 48 is enlarged to 1.2 times or more,preferably to 1.5 to 3.5 time, and more desirably to 1.5 to 2.0 times inthe direction toward slurry introducing port 50.

In another embodiment, slurry introducing pipe 48 preferably has a shapein which horizontal portion L having an enlarged diameter is 1 m long ormore as shown in FIG. 4, and guide pipe 52 opened to the liquid surfacein a tower is provided inside the slurry introducing port. Turbulence offlow due to sudden volume expansion of a slurry can be eased byproviding a horizontal portion having an enlarged diameter longer than apredetermined length as described above, and vaporization of VCM in anintroducing pipe can be repressed by the weight of a slurry itself.Also, it is possible to selectively introduce a slurry onto the liquidsurface on a perforated plate to prevent its dispersion, and to let thevaporized VCM readily escape in a gas phase portion in a tower byproviding guide pipe 52. In this connection, the slurry introducing pipemay be provided at an angle toward a slurry introducing port so that aslurry is hardly retained.

Further, in the slurry introducing portion, hot water ejecting means 51is provided between slurry introducing port 50 and on-off valve 49. Hotwater ejecting means 51 comprises a hot water pipe provided through theside portion of a slurry introducing port, and a spray nozzle providedat the tip thereof. The spray nozzle is preferably arranged with thenozzle being directed toward an on-off valve so that the PVC in the deadspace between an on-off valve and a slurry introducing port can bewashed away.

On-off valve 49 in a PVC slurry introducing portion preferably has astructure in which a PVC slurry is not retained in a valve and PVCslurry introducing portion. While there are no specific restrictions sofar as the valve has such structure, its structure is preferably ballvalve in particular. If it is a ball valve, a PVC slurry does not remainin the valve, PVC particles do not stay in a PVC slurry introducingportion even when the grade of a PVC slurry to be treated is changed,and thus such excellent results that fish eyes due to mixing ofdifferent grades do not occur in final PVC molded articles can beobtained. With regard to the position of the valve, when it is providedat a position as close to a tower as possible, a slurry does not remainbetween a PVC slurry introducing port and the valve, and thus betterresults are obtained.

Further, such hot water washing apparatus 51 as described above can beprovided in a PVC slurry introducing portion to id avoid mixing ofdifferent grades when a PVC slurry to be treated is changed. Occurrenceof fish eyes due to mixing of different grades is decreased by washing aPVC slurry introducing port with the hot water washing apparatus at thetime of changing grade or during operation for removing residualmonomers.

Also, remaining of a PVC slurry for long time in a slurry introducingportion can be avoided by washing a slurry introducing portion which isnot being used for introducing a PVC slurry, at the time of operation ofan apparatus for removing residual monomers.

Perforated plates 32 to 38 having many small holes are provided withseveral partition walls vertically set up on each surface of the plates,and form a chambers (spaces) between the lower surface of upperperforated plates and them. The small holes of the perforated plates arebored so that a demonomerization treatment is conducted with steamejected from the perforations when a slurry flows on the perforatedplates.

Size of the small holes is decided in consideration of steam pressureand the amount of steam to be introduced so that a PVC slurry does notflow down through small holes, the small holes are not blockaded, andthe steam ejected from a lower position uniformly passes through thesmall holes.

Diameter of the small holes to be bored in the perforated plates is 5 mmor less, preferably 0.5 to 2 mm, and more desirably 0.7 to 1.5 mm.Opening ratio (total area of perforations/area of perforated plate) in aperforated plate is 0.001 to 10%, preferably 0.04 to 4%, and moredesirably 0.2 to 2%.

When the perforation ratio is excessively small, PVC particles suspendedin a PVC slurry which flows on trays made of perforated plates are notsufficiently stirred, PVC particles settle out, and the efficiency ofremoving residual monomers from PVC particles is decreased. Flowabilityof a PVC slurry is also decreased. On the other hand, when the openingratio is too large, a phenomenon in which a PVC slurry flows downthrough the small holes (hereinafter referred to as liquid leakage)occurs, and a large amount of steam is unwillingly wasted to prevent theliquid leakage from the small holes.

Partition walls are to secure on perforated plates treating passagesthrough which a PVC slurry can flow. By providing treating passagesformed by partition walls, a PVC slurry flows on perforated plates for acertain period of time during which the slurry is subjected todemonomerization with the steam supplied from a lower portion. In FIGS.2 and 3, perforated plate 47 is shown on the upper surface of whichpartition walls 39 to 44 are provided alternately.

Residence time of a PVC slurry within an apparatus for removing residualmonomers corresponds to the time in which a PVC slurry passes throughtreating passages on perforated plates. Accordingly, it is sufficientfor long treating time to increase the number of partition walls toextend a treating passage or to raise the partitions walls. Whiletreating passages are decided by the manner of arranging partitionwalls, a meandering form (winding or zigzag form) shown in FIG. 2 ispreferable, and a spiral form, arrow wheel form, or star form (radial)can be selected depending on the circumstances in addition.

While the number of partition walls and width of treating passages onperforated plates are not specifically restricted, it is not desirablethat the number of the partition wall is excessively increased or thewidth of the treating passages is made too small, because the liquidsurface of a flowing PVC slurry rises to overflow the partition walls,PVC slurries which are different in residence time are mixed, and thusqualities of products are deteriorated.

The apparatus of the present invention has steam introducing port 10 atthe bottom chamber 9 a tower, and for instance, a PVC slurry from steamintroducing port 10 is blown, through the small holes of perforatedplates, in the PVC slurry which is flowing on perforated plates. Theamount of steam introduced at this time is 1-100 kg/h and preferably 5to 50 kg/h per 1 m³ of a PVC slurry. When the amount of steam to beintroduced is too small, it becomes impossible to efficiently removeresidual monomers in a PVC slurry since PVC particles in the PVC slurrysettle out. On the other hand, when the amount of steam to be introducedis too large, dispersion of a PVC slurry becomes vigorous and sometimesflooding occurs. Besides, the effect of removing residual monomers in aPVC slurry is not increased for the large amount of steam introduced.

When the temperature of a PVC slurry is high, the efficiency of removingresidual monomers is increased. However, when the temperature is toohigh, it causes coloring of PVC particles due to thermal degradation,and deteriorates the qualities. Accordingly, adjustment of thetemperature of a PVC slurry links to production of a PVC having highqualities. Generally it is desirable to adjust the temperature of steamand the amount of steam to be introduced so that the temperature of theslurry which flows on perforated plates is 50 to 150° C., preferably 70to 120° C., and more desirably 80 to 110° C.

Pressure inside a tower 4 of an apparatus for removing residual monomersis desirably maintained at 0.2 to 3 kg/cm² (abs).

In the tower 4 of an apparatus of the present invention for removingresidual monomers, at least one hot water ejecting apparatus isprovided. In the apparatus shown in FIG. 1, hot water ejecting means 25to 31 are constructed by forming a pipe into a predetermined shape, thehot water ejecting means are provided directly under perforated plates31 to 36, and the lower surface of perforated plates and inside wall ofa tower are washed by ejecting hot water at predetermined intervals fromejecting nozzles. While the number of ejecting nozzles and the positionof the nozzle orifices are not specifically restricted, it is preferableto arrange the orifices so that the hot water is ejected from theejecting nozzles in the range of 10 to 60° of crossing angle relative tothe vertical line.

Plane shape of the pipe of hot water ejecting apparatuses 25 to 31 isusually Ω or φ form of Greek letter, spiral form, star form, or ameandering form (winding or zigzag form). It may be a type of multiplerings having the same center. It is sufficient that the hot waterejecting apparatuses 25 to 31 are provided in parallel with perforatedplates and can be put in a tower. However, the hot water ejectingapparatuses 25 to 31 are favorably provided so that their outer diametercome to the distance 20 mm or more apart inwardly from the inside wallof a tower, since if they were placed too close to the inside wall of atower, the clearances come to be in danger of being blockaded withwashed away PVC particles and the likes. Outer diameter of the hot waterejecting apparatuses 25 to 31 is preferably 150 to 8,000 mm.

As to the shape of ejecting nozzle orifices provided in hot waterejecting apparatuses 25 to 31, a suitable shape such as a circle,ellipse, and slit can be selected depending on the purpose of use. Inthis connection, the diameter of the circular orifices or the diameterat the maximum equator of the elliptic orifices can usually be selectedfrom 1 to 8 mm, and the maximum length of the slit can also be selectedfrom 1 to 8 mm.

The PVC slurry from which residual monomers were removed by apparatus 4for removing residual monomers is led to heat exchanger 3 with pump 5,cooled by heat exchange, temporarily stored in PVC slurry tank 6, andthen fed to a drying apparatus (not shown in Figs.) through dehydrationstep. Monomer gas removed in an apparatus for removing residual monomerspasses through deaerating port 11 at a top portion of a tower, steam iscondensed and separated from the gas in condenser 7, and then the gas istransferred to a liquefaction and recovery step. In this step, when alarge amount of vinyl chloride monomers are contained in condensed watercondensed in condenser 7, the condensed water may be introduced into anapparatus for removing residual monomers and treated again.

The present invention will next be described in more specifically by wayof Examples and Comparative Examples. However, the scope of the presentinvention is by no means limited by such specific examples.

In the following Examples and Comparative Examples, evaluations weremade as follows:

(1) Method for Determining Concentration of Residual Monomers

A PVC slurry which was subjected to a treatment for removing residualmonomers and discharged from PVC slurry discharging port 12 was sampledand dehydrated. Subsequently, the concentration of residual vinylchloride monomers in vinyl chloride polymers in the range of ppb wasdetermined by Head Space method using a gas chromatograph 9A (tradename) produced by Shimadzu Corp. Determination conditions were inaccordance with those of ASTM D4443, and FID was used for the detectingsection.

(2) Method for Determining Thermal Degradation

A PVC slurry before and after removal of residual monomers weredehydrated, respectively, dried at 40° C. for 24 hours, molded into PVCplates with the following composition and under the following rollconditions, and then degree of thermal degradation was determinedaccording to the method of JIS K7105. It indicates that the larger themeasurement value, the larger the thermal degradation.

Composition: PVC 100 parts Tri-basic lead sulfate 3 parts Di-basic leadsulfate 1 part Calcium stearate 1 part Stearic acid 0.5 part Rollconditions: Roll type 8 inch roll Roll temperature 170° C. Rolling time15 minutes Thickness of rolled sheet 0.32 mm

(3) Method for Evaluating Fish Eyes

A PVC film prepared by processing, under the following conditions, a PVCresin obtained by drying a PVC slurry which was subjected to a treatmentfor removing residual monomers and sampled from sampling port 60 in FIG.1, was divided into five sections of 100 cm² each, number of fish eyesin each of the sections was counted, and the average of the numbers wastaken.

Composition: PVC 100 parts Dioctyl phthalate 45 parts Lead typestabilizer 4 parts Roll conditions: Roll type 6 inch roll Rolltemperature 150° C. Rolling time 5 minutes Thickness of rolled sheet0.32 mm

(4) Method for Evaluating Adhesion of PVC Particles on the Wall of aTower for Removing Residual Monomers

After the operation for the same grade with a tower for removingresidual monomers for 48 hours in total, the wall surface of thechambers in which a PVC slurry was introduced was observed to confirmthe extent of adhesion of PVC particles. Evaluation was made accordingto the following criteria

A . . . Inside wall of a tower maintained a metallic luster and PVCparticles were not adhered on the wall surface.

B . . . PVC particles were adhered on the wall surface, but they werereadily removed by washing with water.

C . . . PVC particles were adhered on the wall some of which particleswere not removed by washing with water and discolored brown.

D . . . PVC particles were adhered in a layer form and discolored darkbrown.

EXAMPLE 1

The apparatus for removing residual monomers used in Example 1 had astructure similar to that shown in FIGS. 1 to 3, and had the followingspecifications:

a) Tower for removing residual monomers Number of chambers 8 PVC slurryintroducing portions were provided at fourth, fifth, sixth, seventh, andeighth chambers from the bottom. b) Perforated plates Diameter ofplates: 1500 mm Diameter of small holes: 1.3 mm opening ratio: 0.3%(total area of small holes/ area of perforated plate) Partition walls(height): 500 mm Width of treating passages: 200 mm c) Hot waterejecting apparatus Diameter: 900 mm Shape: Ring shape having a pipediameter of 50A (outside diameter 60.5 mm) d) PVC slurry introducingportion Diameter of PVC slurry 80A to 150A introducing pipe: (D/d =150/80) Diameter of PVC slurry 150A introducing port: Type of valve:Ball valve

A PVC slurry (containing 30% by weight of a homopolymer of vinylchloride having an average polymerization degree of 1000 and containing25000 ppm of vinyl chloride monomers) after polymerization reaction wasquickly transferred to slurry tank 1, fed to heat exchanger 3 with pump2 at 20 m³/h, heated, and then introduced into tower 4 for removingresidual monomers as shown in FIG. 1 having seven such perforated platesas shown in FIG. 2 and FIG. 3 through PVC slurry introducing port 22.The PVC slurry flowed on treating passages divided with partition wallson perforated plates 32 to 38 and was subjected to a monomer removaltreatment with steam (106° C., 600 kg/h) ejected from the small holes ofperforated plates. PVC slurry flowing on the perforated plates washeated to 100° C. with the steam, flowed down to a lower perforatedplates through a flow-down pipe, and was discharged from tower 12 forremoving residual monomers through PVC slurry discharging port 12.Subsequently, the PVC slurry was fed to heat exchanger 3 with pump 5,cooled down to 50° C. with the heat exchanger, and then introduced intoPVC slurry tank 6.

Vinyl chloride monomers removed from the PVC resin slurry by contactingwith steam on the perforated plates and reached up to the top of thetower while being accompanied with the steam, led through deaeratingport 11 to condenser 7 where they were separated into vinyl chloridemonomers and condensed water. The separated vinyl chloride monomers werefed to a liquidation and recovery process. When a large amount of vinylchloride monomers were contained in the condensed water, the water wasintroduced again into the apparatus for removing residual monomers fromPVC slurry introducing portions and treated again.

Results are shown in Table 1, and it is seen that residual vinylchloride monomers were capable of being removed down to 230 ppb inExample 1. Yellowing index (Yellowing factor) of the PVC resin was asgood as 2.52. Besides, adhesion of PVC particles on the introducing port22 from which a PVC slurry was introduced and the wall surface ofchamber 56 was not observed.

EXAMPLE 2

A PVC slurry was treated in the same manner as in Example 1 with theexception that a slurry containing 30% by weight of a homopolymer havingan average polymerization degree of 700 and containing 25000 ppm ofvinyl chloride monomers was used as PVC slurry and the slurry wasintroduced from PVC introducing port 19 (chamber 59).

Results are shown in Table 1, and it is seen that residual vinylchloride monomers were capable of being removed down to 320 ppb inExample 2. Yellowing index of the PVC resin was 7.72. Besides, adhesionof PVC particles on the wall surface of chamber 59 was not observed.Further, evaluation of fish eyes was conducted to be extremely as few as7 in 100 cm² of the PVC film.

Comparative Example 1

The same PVC slurry as used in Example 1 was treated with the sameapparatus for removing residual monomers having the same specificationsas in Example 1 with the exception that the apparatus had a slurryintroducing pipe having a pipe diameter of 80A (D/d=1) and a slurryintroducing port having the same diameter only at top chamber of thetower.

Results are shown in Table 1. Whereas residual vinyl chloride monomerswere capable of being removed down to 180 ppb in Comparative Example 1,the yellowing index was 6.84 and became worse than Examples. Besides,PVC particles were adhered on the wall surface of chamber 56 and a partof them was adhered in a layer form.

Comparative Example 2

Comparative Example 1 was repeated by using the same apparatus as usedin Comparative Example 1 with the exception that the same slurry as usedin Example 2 was used.

Results are shown in Table 1. Whereas residual vinyl chloride monomerswere capable of being removed down to 300 ppb in Comparative Example 2,the yellowing index was 8.76 and thermal degradation was progressed thanthat in Example 1. Besides, PVC particles were adhered on the wallsurface of chamber 59 and they were unable to remove even by washingwith water. Further, evaluation of fish eyes was conducted to be asgreat many as 2787 in 100 cm² of the PVC film, and the PVC wasunsuitable as final product.

TABLE 1 Compar- Compar- ative ative Example Example example Example 1 21 2 PVC slurry used (*) [a] [b] [a] [b] Volume of micropores in 0.330.23 0.33 0.23 PVC (ml/g) Number of chambers in 8 8 8 8 tower ChamberNo. in which 5 8 8 8 PVC slurry was introduced (**) Concentration ofresidual 230 320 180 300 mononers in PVC slurry at sampling port (ppb)Yellowing index of PVC 2.52 7.72 6.84 8.76 molded product (ΔYI) State ofadhesion of PVC A A D C particles in PVC slurry introducing chamberafter operation of 48 hours Number of fish eyes — 7 — 2787 (number/100cm²) (*) PVC slurry used: [a] . . . Homopolymer of vinyl chloride havinga polymerization degree of 1000, slurry concentration 30% by weight,concentration of residual monomers: 27000 ppm [b] . . . Homopolymer ofvinyl chloride having a polymerization degree of 700, slurryconcentration 30% by weight, concentration of residual monomers: 25000ppm (**) Chamber No. in which PVC slurry was introduced: Chamber No. inwhich PVC slurry was introduced was counted from the bottom of a tower.

INDUSTRIAL APPLICABILITY

As will be clear from the results described above, the following effectscan be achieved according to the apparatuses of the present inventionfor removing residual monomers:

(1) Residual monomers can be removed at a high efficiency from a PVCslurry while suppressing the thermal degradation of the PVC at a minimumextent.

(2) When removing residual monomers, contact time of a PVC slurry withsteam can be adjusted to a time suitable to the PVC slurry therebyprevent the PVC from thermal degradation caused by the contact withsteam for longer time than necessary.

(3) Residence time of a PVC slurry can voluntarily be controlled bychanging a PVC slurry introducing portion while maintaining the amountof PVC slurry to be treated for demonomeriaztion constant.

(4) Dispersion of a PVC slurry in the chambers of a treating tower inwhich chambers the slurry is introduced can be prevented to avoidadhesion of PVC particles on the wall surface of the treating tower.

(5) When changing a PVC slurry to be treated, a PVC slurry and PVCparticles hardly remain within a treating tower, and occurrence of fisheyes caused by mixing of a different grade of slurries can be avoided.

What is claimed is:
 1. A method for removing residual monomers from aslurry containing a polyvinyl chloride and residual monomers afterpolymerization, with an apparatus comprising a cylindrical tower whichhas plural perforated plates provided in a vertical direction in thetower, plural chambers formed respectively on the perforated platesserving as the bottom surface of the chambers, slurry introducingdevices provided at at least two of said chambers, flow-down sectionsprovided between the perforated plates so as to allow the slurry tosequentially flow down from the perforated plate of an upper chamber tothe perforated plate of a lower chamber, a steam introducing portprovided at a bottom portion of the tower, a deaerating port provided ata top portion of the tower, a slurry discharging port provided at achamber positioned lower than the chambers having said slurryintroducing devices, and a hot water ejecting means provided directlyunder said perforated plates and directed toward at least the lowersurface of said perforated plates, said slurry introducing devicescomprising slurry introducing ports provided at said tower and slurryintroducing pipes connected to the slurry introducing ports, the insidediameter of the slurry introducing pipes being enlarged at least 1.2times toward the slurry introducing ports, which method comprisesintroducing the slurry into the tower in an amount of 0.1 to 300 m³/hper 1 m² of the area of the perforated plates forming the bottom surfaceof said chambers, the linear velocity of the slurry at the slurryintroducing port of the chamber being 6 m/sec or lower when the slurryis introduced from said slurry introducing devices, flowing the slurrydown from at least one perforated plate, and discharging the slurryoutside the tower.
 2. An apparatus for removing residual monomers from aslurry containing a polyvinyl chloride and residual monomers afterpolymerization, comprising a cylindrical tower which has pluralperforated plates provided in a vertical direction in the tower, pluralchambers formed respectively on the perforated plates serving as thebottom surface of the chambers, slurry introducing devices provided atat least two of said chambers, flow-down sections provided between theperforated plates so as to allow the slurry to sequentially flow downfrom the perforated plate of an upper chamber to the perforated plate ofa lower chamber, a steam introducing port provided at a bottom portionof the tower, a deaerating port provided at a top portion of the tower,a slurry discharging port provided at a chamber positioned lower thanthe chambers having said slurry introducing devices, and a hot waterejecting means provided directly under said perforated plates anddirected toward at least the lower surface of said perforated plates,wherein said slurry introducing devices comprise slurry introducingports provided at said tower and slurry introducing pipes connected tothe slurry introducing ports, and the inside diameter of the slurryintroducing pipes is enlarged at least 1.2 times toward the slurryintroducing ports.
 3. The apparatus for removing residual monomersaccording to claim 2 wherein an on-off valve is provided at the slurryintroducing device provided at a chamber positioned below the uppermostchamber.
 4. The apparatus for removing residual monomers according toclaim 3 wherein the on-off valve is a ball valve.
 5. The apparatus forremoving residual monomers according to claim 3 wherein a hot waterejecting means is provided between the slurry introducing port and theon-off valve in the slurry introducing device with the hot waterejecting means being directed toward the on-off valve.